Permanent damage to the hair cells of the inner ear results in sensorineural hearing loss, leading to communication difficulties in a large percentage of the population. Hair cells are the receptor cells that transduce the acoustic stimulus. Regeneration of damaged hair cells provide an avenue for the treatment of a condition that currently has no therapies other than prosthetic devices. Although hair cells do not regenerate in the mammalian cochlea, new hair cells in lower vertebrates are generated from epithelial cells, called supporting cells, that surround hair cells.
The prevalence of hearing loss after damage to the mammalian cochlea has been thought to be due to a lack of spontaneous regeneration of hair cells and/or neurons, the primary components to detect sound (Wong and Ryan, 2015). Humans are born with about 15,000 inner ear hair cells and hair cells do not regenerate after birth. Supporting cells, which surround hair cells in the normal cochlear epithelium, have potential to differentiate into new hair cells in the neonatal mouse following ototoxic damage (Bramhall et al. 2014). Using lineage tracing, the new hair cells, predominantly outer hair cells, have been shown to arise from Lgr5-expressing inner pillar and third Deiters cells, and new hair cell generation has been shown to incrementally be increased by pharmacological inhibition of Notch (Bramhall et al. 2014, Mizutari et al. 2014). It has been postulated that the neonatal mammalian cochlea has some capacity for hair cell regeneration following damage alone (Cox et al. 2014) and that Lgr5-positive (Lgr5+) cells act as hair cell progenitors in the cochlea (Chai et al. 2011, Shi et al. 2012).
Auditory dysfunction in humans is an ongoing problem in the medical fields of otology and audiology. Auditory dysfunctions typically arise from both acute and chronic exposures to loud sounds, ototoxic chemicals, and aging. Sounds exceeding 85 decibels can cause hearing loss and is generated by sound sources such as, gun shots, exploding bombs, jet engines, power tools, and musical concerts. Other common everyday activities and products also give rise to high intensity noise such as use of hair dryers, MP3 players, lawn mowers, and blenders. Military personnel are particularly at risk for noise induced hearing loss due to typical military noise exposures. Side effects of noise-induced hearing loss include tinnitus (ringing in the ears), diminished speech understanding, hyperacusis, recruitment and various types of auditory processing impairments. Exposures to commonly used medications may also induce auditory dysfunctions. For instance, patients treated with anticancer therapies, antibiotics and other medications often develop hearing loss as a side effect. Furthermore, exposure to industrial chemicals and gasses may induce auditory impairments. Auditory dysfunction is a common consequence of aging in Western societies. Hearing impairments can be attributed to a wide variety of causes, including infections (e.g., otitis media), genetic predisposition, mechanical injury, tumors, loud sounds or prolonged exposure to noise, aging, and chemical-induced ototoxicity (e.g., antibiotics or platin drugs) that damages neurons and/or hair cells of the peripheral auditory system. This can be caused by acute noise or can be progressive over time.
Currently, very few cases of hearing loss can actually be cured. Audiological devices such as hearing aids have limitations including the inability to improve speech intelligibility. Of those impacted by hearing impairments, less than 20 percent presently use hearing instruments. In cases of age-related, noise- or drug-induced auditory dysfunctions, often the only effective way to currently “treat” the disorder or reduce its severity is prevention: avoiding excessive noise and using ear protectors, practicing a healthy lifestyle, and avoiding exposure to ototoxic drugs and substances if possible.
Once the hearing loss has developed, people may use a hearing aid to correct the hearing loss. However, despite advances in the performance of these prostheses, they still have significant limitations. For example, hearing aids mainly amplify sound and cannot correct for suprathreshold or retrocochlear impairments such as impaired speech intelligibility, speech in noise deficits, tinnitus, hyperacusis, loudness recruitment and various other types of central auditory processing disorders. Hearing aids essentially amplify sounds, which stimulate unimpaired cells, but there is no therapy for aiding recovery of impaired cells or maximizing the function of existing unimpaired cells.
In cases of complete or profound deafness, a cochlear implant may be used. This device transmits electrical stimuli via electrodes surgically implanted into the cochlea. A cochlear implant can be of particular help for deaf children if it is implanted around the age of two or three, the time when language skills are developing fastest. However, cochlear implants involve invasive surgery and are expensive. Furthermore, cochlear implants require viable neurons to achieve benefit.
Approximately 17 percent of Americans have hearing loss and half of that number are under the age of 65. It is predicted that the number of Americans with hearing loss will exceed 70 million by the year 2030.
About 300 million people worldwide currently suffer from moderate to severe hearing loss, and this number is expected to increase to 700 million by the year 2015. Most of these people will suffer from noise induced hearing loss and one in four Americans will develop permanent hearing loss as a result of occupational exposure to noise hazards. According to the Center for Commercialization of Advanced Technology, the Department of Defense and the VA, the VA spends over $1 billion on hearing loss compensation. The Navy, Marine Corps, and Air Force (combined) file 22,000 new hearing loss claims, and hearing loss costs the economy more than $56 billion per year.
Thus, there remains a long felt need to protect auditory cells before injury and preserve/promote the function of existing cells after injury. As disclosed below, in certain embodiments, the present invention provides compositions, systems, and methods for preventing and treating auditory dysfunctions.
There are many patient populations that could be helped with new therapies that prevent or treat hearing loss, for example, patients with vertigo, tinnitus, or patients who require a cochlear implant, those who have hearing loss but are not eligible for a cochlear implants, and those with chronic mild/moderate or severe hearing loss.
Previous work has shown that manipulating direct inhibitors of cycle activation (e.g., p27kip1, Rb1, p19ink4d, p21cip1) causes many cells, including hair cells, to proliferate. Hair cells that re-enter cell cycle subsequently die and hearing ability deteriorates (Salvi R. J. Hair Cell Regeneration Repair and Protection, Sage et al 2005, 2006). Differentiation to hair cells was not seen after supporting cell proliferation resulting from manipulation of cell cycle genes, such as p27Kip1 or Rb (Yu et al. 2010, Liu et al. 2012).
Stem cells exhibit an extraordinary ability to generate multiple cell types in the body. Besides embryonic stem cells, tissue specific stem cells serve a critical role during development as well as in homeostasis and injury repair in the adult. Stem cells renew themselves through proliferation as well as generate tissue specific cell types through differentiation. The characteristics of different stem cells varies from tissue to tissue, and are determined by their intrinsic genetic and epigenetic status. However, the balance between self-renewal and differentiation of different stem cells are all stringently controlled. Uncontrolled self-renewal may lead to overgrowth of stem cells and possibly tumor formation, while uncontrolled differentiation may exhaust the stem cell pool, leading to an impaired ability to sustain tissue homeostasis. Thus, stem cells continuously sense their environment and appropriately respond with proliferation, differentiation or apoptosis. It would be desirable to drive regeneration by controlling the timing and extent of stem cell proliferation and differentiation. Controlling the proliferation with small molecules that are cleared over time would allow for control of the timing and extent of stem cell proliferation and differentiation. Remarkably, tissue stem cells from different tissues share a limited number of signaling pathways for the regulation of their self-renewal and differentiation, albeit in a very context dependent manner. One of these pathways is the Notch pathway.
The Notch pathway represents an evolutionarily conserved signaling pathway that possesses a simple but unique mode of action. The core Notch pathway contains only a small number of components. The canonical Notch pathway is activated through the binding of Notch ligand on the surface of signal-sending cells to the Notch receptor on neighbor signal-receiving cells. This event initiates a cascade of proteolytic cleavages of the Notch receptor, including γ-secretase-mediated release of the Notch Intracellular Domain (NICD). NICD fragment then enters the nucleus to induce target gene transcription. Under most circumstances, the canonical Notch pathway requires physical contact between neighboring cells, thus it links the fate of one cell to that of an immediate neighbor, providing a sophisticated way to control the self-renewal and differentiation of stem cells. The Notch pathway has been shown to regulate many types of stem cells, including embryonic stem cells, neural stem cells, hematopoietic stem cells as well as Lgr5 epithelial stem cells (Koch et al, 2013; VanDussen et al, 2012).
Lgr5 is expressed across a diverse range of tissues and has been identified as a biomarker of adult stem cells in certain tissues such as the gut epithelia (Barker et al. 2007), kidney, hair follicle, and stomach (Barker et al, 2010; Haegebarth & Clevers, 2009). It was first published in 2011, that mammalian inner ear hair cells are derived from LGR5+ cells (Chai et al, 2011, Shi et al. 2012). Lgr5 is a known component of the Wnt/beta-catenin pathway, which has been shown to play major roles in differentiation, proliferation, and inducing stem cell characteristics (Barker et al. 2007).
Prior work has focused on transdifferentiation of supporting cells into hair cells through activation or forced expression of genes that lead to hair cell formation, with a particular focus on mechanisms to enhance expression of Atoh1 (Bermingham et al., 1999; Zheng and Gao, 2000; Izumikawa et al., 2005; Mizutari et al., 2013). Interestingly, cells transduced with Atoh1 vectors have been shown to acquire vestibular phenotypes (Kawamoto et al., 2003; Huang et al., 2009; Yang et al., 2012, 2013), and lack complete development. As mentioned, upregulating Atoh1 via gene insertion has been shown to create non-cochlear cell types that behave in a manner that is not found within the native cochlea. In addition, these methods increase hair cell numbers but decrease supporting cell numbers. Since supporting cells are known to have specialized roles (Ramirez-Camancho 2006, Dale and Jagger 2010), loss of these cells could create problems in proper cochlear function.